If you search all the trees that have been mapped as areas and look for the biggest ones (in terms of geometry), all you find is a lot of objects where people presumably meant to type natural=wood. All from JOSM it seems - not sure if there’s a specific reason.
The tree that is actually the biggest in the world, by area, has been double mapped as a natural=tree node and a natural=wood area, so it won’t be found with this query…